Decoating and paint stripping are specific examples of removing preselected types and amounts of materials from a substrate or structure. Removal of paint from buildings, automobiles, bridges, aircraft and other structures is a very large industry throughout the world. Refurbishment of paint on aircraft, for example, is a matter of routine maintenance with an average coat of paint lasting typically between about 3 and about 5 years. Furthermore, removal or rust and corrosion of all types, including fouling on the undersides of maritime boats and vessels, is also time-consuming, expensive and difficult.
Problems associated with decoating or paint removal are various. In certain instances, recessed or angular areas or areas with otherwise inaccessible surfaces are difficult to treat. Mechanical methods of decoating include manual or assisted sanding and scraping, grinding with an electric grinder, and blasting the surface with particles such as metal or plastic pellets, sand, etc. Drawbacks of these methods include the possible damage to underlying substrate structure. The upper layer of substrate or structure is often slightly pitted or otherwise damaged and stress hardening is known to occur. These changes in substrate or structure integrity or quality can have a negative impact on the function or utility of the structure. For example, smooth flat surfaces are aesthetically pleasing on an automobile's painted surfaces. The design of an aircraft may be based on a given construction material's physical characteristics such as static and dynamic strength, shear strength, or modulus of elasticity, and changing these physical characteristics could have a negative impact on the given material's performance under certain situations.
Chemical methods of removing coatings or other types of materials are similarly fraught with a range of drawbacks. The primary requirement is chemical solubility of the material to be removed and essential insolubility of the substrate or structure--a situation often difficult to achieve. Particular care must be taken when the substrate or structure is plastic or epoxy-based since these and other organic materials are easily affected by certain solvents. Furthermore, chemical processes for decoating, paint-stripping and the like often pose a significant risk to human and animal health and our environment either through direct exposure to employees during the process or during waste treatment and down-stream processing, industrial accidents, etc.
Methods and apparatus for removing material, in particular for multi-purpose decoating, stripping or scaling, based upon delivery of broadband, non-coherent electromagnetic energy such as that produced by a Xenon-gas or other type flashlamp for rapidly and efficiently vaporizing the material to be removed is heretofore unknown. Although laser energy has been used to in hundreds of applications to vaporize or ablate all types of materials including metals, human tissue and semiconductors, laser devices are dangerous, expensive and often awkward to use. U.S. Pat. No. 4,867,796 to Asmus et al. discusses a method for photo-decontamination of surfaces in which broad band frequency light absorption agents are admixed into the contaminants which are then irradiated. As with a laser, as the light-absorptive agent absorbs the radiation it becomes hot and vaporizes along with the surface decontaminant.
Numerous methods for controlling a flashlamp-type decoating apparatus have been proposed. U.S. Pat. No. 5,281,798 to Hamm et al. proposes a method and system for selectively removing a material coating from a substrate or structure using a flashlamp in conjunction with a photodetector circuit for sensing reflected light back from the surface being ablated, thereby providing a feedback signal indicative of the reflected color intensity of the surface being ablated. The method is dependent upon a boundary between the coatings or between the coating and the substrate or structure to be distinguished by a visible color change, requires additional optical and processing means for photodetection and signal processing.
U.S. Pat. No. 5,194,723 to Cates et al. proposes another method for controlling a pulsed light material removal system based upon detection of "photoacoustic signature signals" generated by the ablation of surface material. However, the complicated method requires sophisticated robotics and sensitive photoacoustic hardware for calibration which involves initial scanning of a structure along a predetermined path, detecting the photoacoustic signature signals, determining an updated scan speed functionally related to the detected photoacoustic signals and controlling the scan speed thereby. Furthermore, the method is dependent on the measurement of so-called photoacoustic signature signals which may or may not exist or be detectable at the required operating temperatures and for the broad range of materials for which a flashlamp-type decoating system would be desirable.
In yet another attempt to overcome the problems associated with using pulsed or continuous wave light energy systems for decoating U.S. Pat. No. 5,204,517 also to Cates et al. theorizes a method and system for control of a material removal process using "spectral emission discrimination". This system utilizes, in addition to photoablative light beam radiation from a flashlamp to remove particles from the surface of the substrate or structure, laser light beam radiation which also ablates material at the surface of the coated substrate or structure so that such ablating material generates spectral emission which can be detected with additional tuned photo-optics or sensors without interference from the spectral output of the flashlamp. In all controls schemes for such flashlamp-type decoating systems proposed in the past, complicated optics and sensor equipment including sophisticated and expensive arrays of diodes with required signal processing hardware and software is required.
Thus, broadly, it is an advantage of the present invention to provide a near-blackbody radiator means for removal of material method and apparatus, said near-blackbody radiator means comprising a highly efficient flashlamp or other type high peak power, high average power, broadband, continuum output radiation for rapidly and efficiently vaporizing the material to be removed.
It is a further advantage of the present invention to provide such a method and apparatus which does not contribute to the production of toxic or other chemical waste.
It is a further advantage of the present invention to provide such a method and apparatus which decontaminates or otherwise reduces the removed material to elemental form or other inert, non-reactive, non-toxic, environmentally friendly soot, ash or other form of debris.
Another drawback of the prior art is that no control scheme based upon required pulse duration is available for removing paint, other coatings or other material from a structure. It is an advantage and objective of the present invention to provide a method based on a pulse duration-control scheme for controllably removing predetermined thicknesses of material from a structure.